1. Field of the Invention
This invention relates to catalyst compositions, process wherein these compositions are used for the preparation of liquid hydrocarbons from synthesis gas, and process for the preparation of said catalysts. In particular, it relates to catalysts, and process wherein C.sub.10+ distillate fuels, and other valuable products, are prepared by reaction of carbon monoxide and hydrogen over cobalt catalysts wherein the metal is dispersed as a thin film on the outside surface of a particulate carrier, or support, especially a titania carrier, or support.
2. The Prior Art
Particulate catalysts, as is well known, are normally formed by dispersing catalytically active metals, or the compounds thereof upon carriers, or supports. Generally, in making catalysts the objective is to disperse the catalytically active material as uniformly as possible throughout a particulate porous support, this providing a uniformity of catalytically active sites from the center of a particle outwardly.
Catalysts have also been formed by dispersing catalytically active materials upon dense support particles; particles impervious to penetration by the catalytically active materials. Ceramic or metal cores have been selected to provide better heat transfer characteristics, albeit generally the impervious dense cores of the catalyst particles overconcentrates the catalytically active sites within a reduced reactor space and lessens the effectiveness of the catalyst. Sometimes, even in forming catalysts from porous support particles greater amounts of the catalytic materials are concentrated near the surface of the particles simply because of the inherent difficulty of obtaining more uniform dispersions of the catalytic materials throughout the porous support particles. For example, a catalytic component may have such strong affinity for the support surface that it tends to attach to the most immediately accessible surface and cannot be easily displaced and transported to a more central location within the particle. Catalyst dispersion aids, or agents are for this reason often used to overcome this effect and obtain better and more uniform dispersion of the catalytically active material throughout the catalyst particles.
Fischer-Tropsch synthesis for the production of hydrocarbons from carbon monoxide and hydrogen is now well known, and described in the technical and patent literature. The earlier Fischer-Tropsch catalysts were constituted for the most part of non-noble metals dispersed throughout a porous inorganic oxide support. The Group VIII non-noble metals, iron, cobalt, and nickel have been widely used in Fischer-Tropsch reactions, and these metals have been promoted with various other metals, and supported in various ways on various substrates, principally alumina. Most commercial experience, however, has been based on cobalt and iron catalysts. The first commercial Fischer-Tropsch operation utilized a cobalt catalyst, though later more active iron catalysts were also commercialized. The cobalt and iron catalysts were formed by compositing the metal throughout an inorganic oxide support. An important advance in Fischer-Tropsch catalysts occurred with the use of nickel-thoria on kieselguhr in the early thirties. This catalyst was followed within a year by the corresponding cobalt catalyst, 100 Co:18 ThO.sub.2 :100 kieselguhr, parts by weight, and over the next few years by catalysts constituted of 100 Co:18 ThO.sub.2 :200 kieselguhr and 100 Co:5 ThO.sub.2 :8 MgO:200 kieselguhr, respectively. These early cobalt catalysts, however, are of generally low activity necessitating a multiple staged process, as well as low synthesis gas throughput. The iron catalysts, on the other hand, are not really suitable for natural gas conversion due to the high degree of water gas shift activity possessed by iron catalysts. Thus, more of the synthesis gas is converted to carbon dioxide in accordance with the equation: H.sub.2 +2CO.fwdarw.(CH.sub.2).sub.X +CO.sub.2 ; with too little of the synthesis gas being converted to hydrocarbons and water as in the more desirable reaction, represented by the equation: 2H.sub.2 +CO.fwdarw.(CH.sub.2).sub.X +H.sub.2 O.
U.S. Pat. No. 4,542,122 by Payne et al, which issued Sep. 17, 1985, describes improved cobalt catalyst compositions useful for the preparation of liquid hydrocarbons from synthesis gas. These catalyst compositions are characterized, in particular, as cobalt-titania or thoria promoted cobalt-titania, wherein cobalt, or cobalt and thoria, is composited or dispersed upon titania, or titania-containing support, especially a high rutile content titania. U.S. Pat. No. 4,568,663 by Mauldin, which issued Feb. 4, 1986, also discloses cobalt-titania catalysts to which rhenium is added to improve catalyst activity, and regeneration stability. These catalysts have performed admirably well in conducting Fischer-Tropsch reactions, and in contrast to earlier cobalt catalysts provide high liquid hydrocarbon selectivities, with relatively low methane formation.
Recent European Publication 0 178 008 (based on Application No. 85201546.0, filed: Sep. 25, 1985) and European Publication 0 174 696 (based on Application No. 852011412.5, filed: May 5, 1985), having priority dates Apr. 4, 1984 NL 8403021 and 13.09.84 NL 8402807, respectively, also disclose cobalt catalysts as well as a process for the preparation of such catalysts by immersion of a porous carrier once or repetitively within a solution containing a cobalt compound. The cobalt is dispersed over the porous carrier to satisfy the relation .SIGMA.V.sub.p /.SIGMA.V.sub.c .ltoreq.0.85 and .SIGMA.V.sub.p /.SIGMA.V.sub.c .ltoreq.0.55, respectively, where .SIGMA.V.sub.c represents the total volume of the catalyst particles and .SIGMA.V.sub.p the peel volumes present in the catalyst particles, the catalyst particles being regarded as constituted of a kernal surrounded by a peel. The kernal is further defined as one of such shape that at every point of the kernal perimeter the shortest distance (d) to the perimeter of the peel is the same, d being equal for all particles under consideration, and having been chosen such that the quantity of cobalt present in .SIGMA.V.sub.p is 90% of the quantity of cobalt present in .SIGMA.V.sub.c. These particular catalysts, it is disclosed, show higher C.sub.5+ selectivities than catalysts otherwise similar except that the cobalt component thereof is homogeneously distributed, or uniformly dispersed, throughout the carrier. Suitable porous carriers are disclosed as silica, alumina, or silica-alumina, and of these silica is preferred. Zirconium, titanium, chromium and ruthenium are disclosed as preferred of a broader group of promoters. Albeit these catalysts may provide better selectivities in synthesis gas conversion reactions vis-a-vis catalysts otherwise similar except the cobalt is uniformly dispersed throughout the carrier, like other cobalt catalysts disclosed in the prior art, the intrinsic activities of these catalysts are too low as a consequence of which higher temperatures are required to obtain a productivity which is desirable for commercial operations. Higher temperature operation, however, leads to a corresponding increase in the methane selectivity and a decrease in the production of the more valuable liquid hydrocarbons.
Productivity, which is defined as the standard volumes of carbon monoxide converted/volume catalyst/hour, is, of course, the life blood of a commercial operation. High productivities are essential in achieving commercially viable operations. However, it is also essential that high productivity be achieved without high methane formation, for methane production results in lower production of liquid hydrocarbons. Accordingly, an important and necessary objective in the production and development of catalysts is to produce catalysts which are capable of high productivity, combined with low methane selectivity.
Despite improvements, there nonetheless remains a need for catalysts capable of increased productivity, without increased methane selectivity. There is, in particular, a need to provide further improved catalysts, and process for the use of these catalysts in synthesis gas conversion reactions, to provided further increased liquid hydrocarbon selectivity, especially C.sub.10+ liquid hydrocarbon selectivity, with further reduced methane formation.